1. Field of the Invention
The present invention relates to a distortion compensating circuit that is usable for a radio transmission device of a base station or the like in a wireless communication system and serves to compensate distortion occurring in amplifying means such as a power amplifier or the like.
2. Description of the Related Art
In order to suppress adjacent channel leakage power, it is ideal for a radio transmission device in a wireless communication system to use a power amplifier having excellent linearity in all the amplitude components. However, if a power amplifier is designed to have excellent linearity to even remarkably large amplitude components, the scale of the power amplifier is large, the price thereof is high and the power consumption is also large. Therefore, there may be such a case that a power amplifier having excellent linearity to small amplitude components, but having non-linearity to large amplitude components must be used. When such a power amplifier is used, the adjacent channel leakage power would be increased if amplitude components having larger power levels than the effective power are amplified.
Therefore, various distortion compensating techniques have been proposed in order to suppress the adjacent channel leakage power due to non-linear distortion of power amplifiers. A feed-forward type, a Cartesian feedback type and a pre-distortion type have been proposed as representative distortion compensating techniques. With respect to the feed-forward type, it is difficult to enhance the efficiency because it structurally needs an error amplifier. Therefore, the distortion compensation based on the Cartesian feedback type or the pre-distortion type with which the distortion compensation on the digital quadrature coordinates in the baseband can be performed have been recently considered from the viewpoint of reduction of the cost and enhancement of the efficiency (see JPA-8-78967, JPA-8-251246, for example).
In the Cartesian feedback type, the distortion improvement amount is proportional to the loop gain, so that it is difficult to increase a distortion-improvable band width in order to enhance the distortion improvement degree and keep the loop stability for negative feedback. Accordingly, the pre-distortion type which can increase the band width is more effective to a mobile communication system, and thus the following description will be made by exemplifying the pre-distortion type.
FIG. 1 is a block diagram showing an example of the construction of a radio transmission device having a pre-distortion type distortion compensating circuit based on a prior art. The conventional radio transmission device comprises transmission data generator 1, D/A converters 31, 32, quadrature modulator 4, reference signal generator 5, power amplifier 6 and distortion compensating circuit 30. The distortion compensating circuit 30 comprises non-linear distortion compensating calculator 2, directional coupler 7, quadrature demodulator 8, A/D converters 91, 92, power calculator 10, and error calculation and compensation data renewing unit 11.
In the non-linear distortion compensating calculator 2, digital quadrature baseband signals I, Q (in-phase signal and quadrature signal) from the transmission data generator 1 are subjected to distortion compensation calculation using complex-multiplication based on distortion compensating data which are calculated in advance. The quadrature baseband signals I′, Q′ after the distortion compensation calculation is carried out are converted to analog signals by the D/A converters 31, 32 to thereby achieve analog quadrature baseband signals. Subsequently, in the quadrature modulator 4, the analog quadrature baseband signals are converted to a quadrature modulation signal on the basis of a signal from the reference signal generator 5. Thereafter, the quadrature modulation signal is subjected to power amplification in the power amplifier 6, and output as an RF output.
A part of the output of the power amplifier 6 is fed back to the quadrature demodulator 8 by the directional coupler 7, and demodulated to the analog quadrature baseband signals on the basis of the signal from the reference signal generator 5. Further, the analog quadrature baseband signals are converted to digital signals by the A/D converters 91, 92 to thereby achieve digital quadrature baseband signals I″ and Q″. The digital quadrature baseband signals I″ and Q″ thus fed back and the input quadrature baseband signals I and Q from the transmission data generator 1 are respectively compared with each other in the error calculation and compensation data renewing unit 11 to renew the distortion compensation data. In the non-linear distortion compensation calculator 2, the distortion compensation data are referred to on the basis of the distortion compensation data thus renewed by using the power value from the power calculator 10 as an address to thereby perform the distortion compensation.
FIG. 2 is a block diagram showing the construction of the error calculation and compensation data renewing unit 11 shown in FIG. 1.
The error calculation and compensation data renewing unit 11 comprises distortion compensation data memory 41 and distortion compensation data calculator 42 as shown in FIG. 2.
The distortion compensation data calculator 42 compares the quadrature baseband transmission signals I, Q and the feedback signals I″, Q″ on the polar coordinates to calculate an amplification error and a phase error. The distortion compensation data memory 41 stores the distortion compensation data calculated in the distortion compensation data calculator 42 while associating the distortion compensation data with the power values.
The construction described above is an example of the prior art, and other constructions using a digital system for quadrature modulation, quadrature demodulation or using a frequency converter without using direct modulation have been proposed.
Further, other construction for carrying the following calculation has been proposed: The distortion compensation calculation is carried out while an amplification value corresponding to the square root of a power value from an amplification calculator is used as an address in place of using the power value from the power calculator 10 as an address.
The conventional pre-distortion type and Cartesian feedback type distortion compensation circuits compensate only non-linear distortion in AM/AM characteristic (amplification characteristic) and AM/PM characteristic (phase characteristic) occurring in the power amplifier 6, etc. Multiplexed modulation waves based on CDMA (code Division Multiple Access) modulation or OFDM (Orthogonal Frequency Division Multiplexing) modulation contain amplitude components in which the instantaneous power of an envelope is remarkably larger than the effective power, and thus clipping distortion occurs in a saturated area. However, in the conventional pre-distortion type and Cartesian feedback type distortion compensation circuits, the clipping distortion components cannot be compensated.
Furthermore, there is also such a disadvantage that the amplitude compensation when the distortion compensation is carried out causes the signal amplitude after compensation to reach the saturated area of the power amplifier for even instantaneous input amplitude components which are lower than the saturation input in the case of non-compensation by several dB (decibel), thereby increasing clipping distortion, so that the distortion compensation increasing clipping distortion, so that the distortion compensation effect is lowered.
A distortion compensation circuit for suppressing very large instantaneous amplitude components of an envelope which causes the above problem has been proposed in JPA-2001-251148.
FIG. 3 shows the construction of an error calculation and compensation data renewing unit of another conventional distortion compensation circuit as disclosed in JP-2001-251148. In FIG. 3, the same constituent elements as shown in FIG. 2 are represented by the same reference numerals, and the description thereof is omitted.
As shown in FIG. 3, the error calculation and compensation data renewing unit 11a comprises distortion compensation data memory 41, distortion compensation data calculator 42, selector 43, comparator 44 and distortion compensation data correcting portion 45.
The comparator 44 compares a preset upper limit power value Pmax and the power value of signals I′, Q′ achieved by conducting the distortion compensation calculation on input quadrature baseband transmission signals I, Q with distortion compensation data calculated in the distortion compensation data calculator 42. The distortion compensation data correcting unit 45 corrects the distortion compensation data calculated in the distortion compensation data calculator 42 so that the amplitude compensation coefficient is equal to 1/m, and outputs the corrected distortion compensation data. Here, m represents the amplitude limiting coefficient, and it is calculated from m=(the power of the quadrature baseband transmission signal after distortion compensation calculation/Pmax)1/2.
If it is judged in the comparator 44 that the power value of the signals I′, Q′ after the distortion compensation calculation is carried out on the input quadrature baseband transmission signal is larger than the power value Pmax, the selector 43 selects the distortion compensation data from the distortion compensation data correcting portion 45 and outputs it to the distortion compensation data memory 41. If it is judged in the comparator 44 that the power of the signals after the distortion compensation is carried out on the input quadrature baseband transmission signal is not more than the power value Pmax, the selector 43 selects the distortion compensation data from the distortion compensation data calculator 42 and outputs it to the distortion compensation data memory 41.
When the power value of the quadrature baseband signals I′, Q′ after the distortion compensation calculation is larger than the fixed value Pmax, the other conventional distortion circuit described above can correct the magnitude of the amplitude compensation coefficient with keeping the phase to the input signals and limit the signal amplitude after the distortion compensation. However, in this conventional technique, the amplitude of the quadrature baseband signals I, Q input to the distortion compensation data calculator 42 is not limited because the amplitude compensation coefficient itself in the non-linear distortion compensation calculator 2 is multiplied by the amplitude limiting coefficient for amplitude limitation and then stored in the distortion compensation data memory 41. Accordingly, in the construction of the conventional technique described above, the amplitude-limited feedback signal from the power amplifier 6 and the amplitude non-limited quadrature baseband signal are compared with each other to calculate the distortion compensation data. Therefore, by the conventional distortion compensation circuit having the construction as described above, accurate error correction cannot be performed and the distortion compensation cannot be performed with high precision.
The conventional distortion compensation circuit as described above has the disadvantage that the accurate error calculation cannot be performed because the input quadrature baseband signal before the amplitude limitation is carried out and the feedback signal after the amplitude limitation is carried out are compared with each other to perform the error calculation.